1. Field of the Invention
The present invention relates to fiber reinforced composites made of inorganic sinterable material and inorganic fibers and a process for the manufacture thereof.
2. Description of the Prior Art
In search of materials suitable for heat resistant structural components which are especially used in aeronautics and astronautics, and engine and turbine manufacturing, one meets with limitations in developing alloy materials. On the one hand, it is difficult to achieve the desired heat resistance at temperatures above 800.degree. C. On the other hand, for these materials, raw materials are required which are rare and expensive, so that the ratio of costs to yield becomes more and more unfavorable. Furthermore, since alloys on the basis of Fe, Ni and Co have a high specific weight and are often used in moving parts, there is beyond that a general interest in developing heat resistant materials with less specific weight.
As materials which can substitute for the alloys in the mentioned fields of application in principle ceramic materials are taken into consideration which excel by having low specific weight and superior temperature resistance under a great variety of atmospheres. In addition, these materials often possess a superior wear resistance as well as a good chemical resistance. Finally, the raw materials involved are available in a sufficient extent and moderately priced.
The fact that ceramic materials are rarely used in spite of these outstanding combinations of properties in the considered fields of application, is due to their typical brittle fracture behavior. The risk that a structural unit will suddenly break catastrophically is regarded as too high.
On grounds of this situation, for 20 years attempts have been made to reduce the brittleness of ceramic materials by the development of composites. In this connection, the development of fiber reinforced glasses and glass ceramics has become an important field of technology. Thereby, increases in bending strength and fraction toughness up to values of about 1000 MPa and 20 MPa.times..sqroot.m, respectively (m=meter) were achieved. The increase of the values of fracture toughness, which is of special significance for the reduction of the brittle fracture behavior, has resulted up to now mainly from experimental experience. The knowledge how fracture development influences the value of the fracture toughness is still lacking.
Up to now in the relevant literature two basically different methods for the manufacture of composites with a glassy or glass ceramic matrix, respectively, are known.
Two methods are suspension technique and the sol gel technique.
In case of the relatively simple, predominantly used suspension method, a matrix powder is mixed with a binder, mostly a mixture of alcohol(s) and a latex binder. The fibers are passed through the matrix/binder bath and thereby impregnated with matrix material, as was first described in 1972 (Sambell et al. (1972), J. Mat. Sci. 7, p. 676 to 681).
In U.S. Pat. No. 4,256,378, a mirror with particularly good properties made of a composite of C-fibers and borosilicate glass is described. The particularly good properties are especially the low relative deformation in extension and the specific rigidity. The production proceeds via the production of a prepreg. Generally, a prepreg is a half finished layer of a multitude of fibers being impregnated with matrix material and being deposited side-by-side in mutual contact with each other. Also a few fiber layers can be deposited one upon the other by passing the C-fibers through a suspension consisting of glass in propanol or glass in propanol, polyvinylalcohol and a wetting agent. Densification of the prepreg is performed by hot pressing under vacuum or an argon protective gas atmosphere at temperatures of 1050.degree. to 1450.degree. C. and pressures between 6.9 and 13.8 MPa. Burning out of binders is not necessary. However, the solvents do not contribute to the properties of the composite material. With respect to a bidirectional reinforced material in a three point transverse bending test, bending strengths up to 387 MPa are described. In a later publication, obviously as a result of process optimization, in a three point transverse bending test bending strengths up to 520 MPa for 35 vol. % SiC-fibers and up to 840 MPa for 50 vol. % SiC-fibers are reported (K. M. Prewo, J. J. Brennan, J. Mat. Sci. 17 (1982), pp. 1201).
In U.S. Pat. No. 4,314,852, the manufacture of composite materials made of SiC-fiber reinforced glasses is described. As glasses, a borosilicate glass, an alkaline earth aluminosilicate glass and a high silica content glass are quoted. The production proceeds via two process steps which are, except for modified amounts of single components, identical with those described in U.S. Pat. No. 4,256,378. With respect to the composite materials very good thermomechanical properties are obtained. With respect to the SiC-fiber reinforced borosilicate glass (SiC-fibers from Nippon Carbon Company, Japan), at room temperature bending strengths of just under 500 MPa for unidirectional and 350 MPa for bidirectional reinforcement are observed. The room temperature bending strength values of SiC-fiber reinforced alkaline earth aluminosilicate glass with a fiber loading of 50 vol. % are about 1000 MPa for bidirectional and just 1400 MPa for unidirectional reinforcement. With respect to the SiC-fiber reinforced high silica content glass, the bending strength values at a fiber loading of 30 to 40 vol. % are between 400 and 550 MPa. With respect to the SiC-fiber reinforced alkaline earth aluminosilicate composites, fracture toughnesses of 16 MPa.times..sqroot.m for bidirectional and 27 MPa.times..sqroot.m for unidirectional reinforcement are reported.
In U.S. Pat. No. 4,485,179, the manufacture of composite materials made of SiC-fiber reinforced glass ceramics is described. As a preferred matrix material, the starting glass (precursor glass) of a Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 -glass ceramic with Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5 and ZrO.sub.2 as nucleation agents is quoted. The production process proceeds as described in U.S. Pat. No. 4,256,378, the suspension consisting of glass, water and latex binder which has to be burned out prior to hot pressing. The conversion of the starting glass (precursor glass) into a glass ceramic can take place either during hot pressing or during an additional temperature treatment. As a particularly important result of material selection, the use of Nb.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5 is emphasized because during the production process NbC or TaC layers, respectively, form on the fibers at the interface. These layers prevent further reaction between the fibers and the matrix at high temperatures and under oxidative conditions. Furthermore, it is stated that the use of TiO.sub.2, which is preferably used as nucleation agent in Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 -glass ceramics, should be completely avoided or at least only used in very small amounts. This is due to the fact that TiO.sub.2 also reacts with SiC-fibers from which, contrary to the reactions between SiC and Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5, a strong degradation of the SiC-fiber properties results. With respect to composite materials with a fiber loading of 50 vol. %, in a three point transverse bending test at room temperature bending strengths of at most just under 1000 MPa are obtained. Fracture toughnesses are not measured, but values above 11 MPa.times..sqroot.m are expected.
The EP 0 126 017 represents a further development of U.S. Pat. No. 4,485,179, in that excellent bending strengths and a good oxidation resistance up to temperatures of 1200.degree. C. are achieved by utilization of another matrix material. The improvements are obtained by using a Ba-modified cordierite or a Ba-osumilite glass ceramic, respectively, instead of the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 -glass ceramic. Powders of the corresponding starting glasses are used as matrix materials, while the conversion of the glass into glass ceramics either takes place during or after hot pressing. As nucleation agents Na.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5 are again used because of their positive effect on the formation of an interface fiber/matrix. At room temperature in a three point transverse bending test bending strengths up to 700 MPa are measured. In especially one example (number 4) wherein a Ba-osumilite glass ceramic matrix is employed, a linear run of the stress strain curve is obtained which is explained by the fact that the matrix has completely crystallized.
The manufacture of fiber reinforced glass ceramics on the basis of stoichiometric cordierite has as yet not been described. Presumably, this is due to the strong tendency of the powders of the starting glasses to crystallize just above the transformation temperature and also due to the difficulties during condensation which result therefrom.
In the state of the art here described as well as in other publications, the suspension bath mostly works according to the fluidized bed principle, as it has been described by Bowen et al. in 1969 (Brit. Pat. Spec. 1,279,252).
Compressed air is injected into the suspension to prevent sedimentation of the powder and to expand the fiber bundles so that they are also impregnated with matrix material in the interior thereof.
The NASA Contr. Reports 158 946 (1978) and 165 711 (1981) mention the addition of 2% LUDOX.RTM. (Trademark of Du Pont) to the binders already described, i.e., the addition of colloidal SiO.sub.2 which leads to an increase in strength.
If binders are used in the above described process, they have to be burnt out after the preparation of the fibers and prior to prepreg densification by means of hot pressing. This means an additional process step. Furthermore, there is a risk that [rests] residues of "alien" binders will remain and contaminate the material. In the case where alcohol is used as the sole binder, the adhesion of the powder to the fibers is poor after its evaporation. This can lead to matrix losses by part of the matrix material dropping off the fibers.
A second method which is quoted in the literature, the sol-gel-technique, is described in the following publications:
Walker et al., Am. Cer. Soc. Bull. 62 (8) (1983), pages 916 to 923;
Rice, Mat. Res. Soc. Sym. Proc. Vol. 32 (1984), pages 337 to 345;
Lannutti, Clark, ebd, pages 369 to 375; and
Lannutti, Clark, ebd, pages 375 to 381.
In this method, the reinforcing fiber is either passed through a sol gel solution of matrix material and then wound up, or it is laid up dry and subsequently impregnated with solution during the course of which the impregnating process can be performed several times.
If composites are to be formed which are non-porous and free of cracks, the partly protracted and complicated drying process of the prepreg (hydrolysis, pyrolysis) represents a disadvantage of this method. High volume shrinkage of the solution is the main problem of the method. In converting the sol into the gel state, the vaporization of the alcohols, normally present in the solution, becomes more and more difficult. If the fibers are laid up wet, there is a risk that in continuously processing the fibers the solution will start to hydrolyze, due to atmospheric moisture, resulting in changes in viscosity.
If the protracted drying process should be avoided, hot pressing can be used for final densification as described by:
Haluska, European Pat. Appl. 0 125 772 (1984);
Fitzer, Schlichting, High Temp. Sci. 13 (1980) pages 149 to 172;
Fitzer, Proc. Int. Fac. in Densification and Sintering of Oxide and Non-Oxide Cer., 1978, Japan; and
Schubert, Diss., University Karlsruhe (1977).